Invasion dynamics of multifocal breast tumor spheroids in three-dimensional extracellular matrix

Breast cancer in human patients often occurs as multifocal lesions, and the mechanisms through which primary tumors in proximity interact with each other is not well understood. Here, we study an in vitro model of bifocal breast tumors by embedding tumor spheroids of MDA-MB-231 cells in 3D extracellular matrices (ECMs) made of type I collagen at different concentrations. Combining quantitative experiment, mathematical modeling, and numerical simulation, we show that collective traction force from the spheroids creates dipolar deformation in the ECM. As a result, in the inter-spheroid region, we see increased ECM alignment and decreased pore size. The effect is particularly pronounced for soft ECM and leads to the spatial gradient of ECM microstructure. Guided by mechanical cues, cancer cells disseminating from spheroids exhibit strong polarization but reduced speed and persistence in the inter-spheroid space. As a result, expansion rate of bifocal spheroids decreases over time, exhibiting self-limiting dynamics. Our results suggest that reciprocal interactions between cancer cells and ECM could mechanically connect multifocal lesions, altering the invasion dynamics of breast cancer in the tissue space.